Triple trouble for mangrove coasts

Mangrove forests, their large biodiversity, and the coastal protection they provide are under pressure from three different threats: sea-level rise, lack of mud and squeezed habitats. A paper featuring new computer simulations, the results of which were published recently in Environmental Research Letters, shows how these coastal forests get pushed against their shores and what causes the loss of their diversity. Landward, river dams have decreased the supply of mud that could raise mangrove soils, while buildings and seawalls largely occupy the space that mangroves require for survival. “Mangrove diversity may be lost even if their total extent increases,” says Danghan Xie, PhD researcher at Utrecht University and lead author of the study.

Mangroves retreat landwards under sea-level rise, but are limited in their retreat by expanding cities, among other factors. Photo: Google Earth.

Coastal mangrove forests are valuable, highly biodiverse ecosystems that protect coastal communities against storms. Mangroves withstand flooding by tides and capture mud to raise the soil level. But as the mangrove trees cannot survive if they are under water for too long, the combination of sea-level rise and the decreasing mud supply from rivers poses a serious threat. 

Landward retreat

New computer simulations show how coastal forests retreat landward under sea-level rise, especially in coastal areas where mud in the water is declining. The simulations include interactions among tides, mud transport and, for the first time, multiple mangrove species. “Both mangrove coverage loss and diversity loss go hand in hand when that landward retreat is limited by expanding cities, agriculture or flood protection works,” says Dr. Barend van Maanen, senior lecturer at the University of Exeter and supervisor of the project.

Mangrove trees with dense roots trap mud more effectively. Photo: Job de Vries.

Narrowing mangrove zone

The model also shows that mangrove trees with dense roots trap mud more effectively and can stop it from reaching forest areas further inland. “This makes landward forests flood for longer periods of time, an effect that is intensified by sea-level rise,” Xie illustrates. “Increased landward flooding then seriously reduces biodiversity in that area. Human land use prevents the mangroves to ‘escape’ flooding by migrating inland, narrowing the mangrove zone and further endangering biodiversity.” A narrow mangrove zone is much less effective in protecting the coast against storms, or in the worst case loses its protective properties altogether.

Ecological and economic implications

Co-author Dr. Christian Schwarz, environmental scientist at the University of Delaware, adds: “The loss of mangrove species will have dramatic ecological and economic implications, but fortunately there are ways to help safeguard these ecosystems. It is essential to secure or restore mud delivery to coasts to counter negative effects of sea level rise. For coasts where mud supply remains limited, removal of barriers that obstruct inland migration is of utmost importance to avoid loss of mangrove forests and biodiversity.” 


The publication Mangrove diversity loss under sea-level rise triggered by bio-morphodynamic feedbacks and anthropogenic pressures was published in Environmental Research Letters. Co-authors are from Utrecht University; University of Delaware, USA; University of Exeter, UK and Hohai University, China. Background and an introduction to this research topic can be found in this video:

Coastal Mangrove Wetlands - Danghan Xie